1. Field of the Invention
The present invention relates to a power supply system, and a control apparatus and a control method thereof, and more particularly to a power supply system that receives a power generation fuel to generate a power generation gas containing hydrogen and supplies the generated gas to a power generating unit to perform a power generating operation, and a control apparatus and a control method thereof.
2. Description of the Related Art
In recent years, with an increase in interest in environmental issues or energy problems, a fuel cell (or a power supply system using a fuel cell) has attracted attention as a power supply system (or a power generating system) that forms a next-generation mainstream. As is well known, a power generation theory of a fuel cell has an advantage that greenhouse gases or contaminants are rarely discharged and an influence on an environment (an environmental load) is very small since an electrochemical reaction using hydrogen and oxygen is utilized to output electrical energy. Further, power generation efficiency (an energy conversion efficiency) that is extremely higher than that in a conventional power generating system (e.g., a system that generates electrical energy based on thermal energy or the like obtained by burning, e.g., a fossil fuel) can be realized, and hence studies and developments aiming at full-scale spread in various industrial fields are actively carried out.
Here, although directly supplying hydrogen gas having a high concentration as a power generation fuel applied to a fuel cell is desirable, a system that directly supplies such hydrogen gas to a fuel cell main body is hard to be extensively and rapidly spread because of difficulties in processing in a technical or safety aspect at the time of generation, storage or supply of the hydrogen gas or an economical viewpoint such as provision of a social infrastructure. This system is put to practical use in a relatively large system, e.g., a power generating unit in a specific business establishment or a driving device for some special vehicles.
On the other hand, when a power supply system using a fuel cell is applied to a small portable electronic device (a portable device), e.g., a notebook computer or a mobile phone, it is considered that using a hydrocarbon-based liquid fuel (alcohols), e.g., methanol or ethanol that is readily available and easy to use and has a low manufacturing cost is realistic.
In a power supply system using such a liquid fuel, as a mode of supplying a power generation fuel to a fuel cell, a direct fuel supply mode of directly supplying a corresponding power generation fuel (methanol) to a fuel cell main body and a reformed fuel supply mode of supplying hydrogen gas obtained by reforming a corresponding power generation fuel are known.
In a fuel cell adopting the direct fuel supply mode, since a power generation fuel such as methanol can be directly supplied to a fuel cell main body, a structure of a later-described fuel reformer or the like is not required in a fuel supply path, and this fuel cell (a power supply system) has an advantage that its structure can be simplified. However, the fuel cell adopting this mode generally has a drawback that its power generation efficiency (an energy conversion efficiency) is lower than that of a fuel cell adopting the reformed fuel supply mode.
On the other hand, the fuel cell adopting the reformed fuel supply mode has an advantage that its power generation efficiency (the energy conversion efficiency) is higher than that of the fuel cell adopting the direct fuel supply mode since hydrogen gas having a high purity (a high concentration) generated by reforming a power generation fuel such as methanol can be supplied to a fuel cell main body.
A power supply system to which a fuel cell adopting the reformed fuel supply mode is applied in a conventional technology will now be briefly explained.
FIG. 6 is a schematic block diagram showing a structural example of the power supply system to which the fuel cell adopting the reformed fuel supply mode is applied in the conventional technology.
FIG. 7 is a schematic view showing an example of a chemical reaction in a chemical reacting section applied to the power supply system to which the fuel cell adopting the reformed fuel supply mode is applied.
As shown in FIG. 6, the power supply system to which the fuel cell in the conventional technology is applied roughly includes a fuel supply section 310 in which a power generation fuel such as methanol is stored or enclosed, a chemical reacting section 320 that reforms the power generation fuel to generate a power generation gas mainly containing hydrogen gas, and a power generation cell section 330 that generates and outputs electrical energy based on an electrochemical reaction using the generated hydrogen gas and oxygen in the atmosphere.
Here, as shown in, e.g., FIGS. 6 and 7, the chemical reacting section 320 includes at least a vaporizer (a fuel vaporizer) 321 that evaporates (vaporizes) an aqueous solution consisting of a power generation fuel (e.g., methanol CH3OH) and water (H2O) to generate a fuel gas, a reformer 322 that modifies the fuel gas based on a reforming reaction to generate a power generation gas containing hydrogen (H2), and a carbon monoxide remover (which will be referred to as a “CO remover” hereinafter) 323 that converts harmful carbon monoxide (CO) in carbon dioxide (CO2) and a small amount of carbon monoxide (CO) produced as byproducts in the reforming reaction into carbon dioxide CO2 based on a selective oxidation reaction and removes the converted carbon dioxide CO2.
In such a structure, when the hydrogen gas having a high concentration generated by the chemical reacting section 320 is supplied to an anode side of the power generation cell section 330, hydrogen ions and electrons are generated from the hydrogen. When the hydrogen ions are transmitted through a proton exchange membrane interposed between the anode and a cathode to be coupled with oxygen molecules (oxygen in the atmosphere) on the cathode side, the electrons that move from the anode side toward the cathode side are output to generate electrical energy. It is to be noted that a specific chemical reaction in the chemical reacting section and the power generation cell section will be explained in detail in a section of “the detailed description of the invention”.
Meanwhile, in the power supply system to which the fuel cell adopting the reformed fuel supply mode is applied, the amount of hydrogen gas supplied from the chemical reacting section 320 to the power generation cell section 330 must be kept constant to stably drive a load connected with the power supply system. Here, a generation state of the hydrogen gas in the chemical reacting section 320 (a progress state of a reforming reaction in the reformer 322) is controlled under temperature conditions set with respect to the chemical reacting section 320 including the reformer 322. Therefore, the chemical reacting section 320 must be set to a predetermined high-temperature state, this state must be maintained, and the progress state of the reforming reaction in the reformer 322 must be held in a predetermined status in order to keep the amount of hydrogen gas generated by the chemical reacting section 320 constant.
Here, as a method of maintaining the chemical reacting section 320 in a predetermined high-temperature state (a fixed temperature), the following technique or the like is known. That is, for example, in a power generating operation (an electrochemical reaction) in the power generation cell section 330, heating is performed by using combustion heat obtained by burning an off-gas containing remaining unreacted hydrogen in a catalyst combustor (not shown) provided near the chemical reacting section 320 or heat obtained from an electric heater (not shown) or the like to set a predetermined high-temperature state.
Specifically, in the technique of setting the chemical reacting section 320 (the reformer 322) to the predetermined high-temperature state (a fixed temperature) by using combustion heat from the off-gas, controlling the amount of off-gas or oxygen supplied to an off-gas combustor allows setting a degree of burning the off-gas (an amount of generated combustion heat), thereby controlling a generation state of the hydrogen gas (a progress state of a reforming reaction) in the chemical reacting section 320 (the reformer 322).
However, a power supply system adopting a structure and a method of controlling a temperature state of the chemical reacting section (the reformer) by using combustion heat from the off-gas is set to supply hydrogen whose amount is larger than a sum total of an amount of hydrogen required for the power generating operation and an amount of hydrogen required as a thermal energy in a hydrogen generating operation in the chemical reacting section (the reformer) in order to stably output fixed electrical energy (a current) based on the power generating operation in the power generation cell section. That is, an amount of hydrogen in the off-gas discharged from the power generation cell section is set to be greatly larger than an amount consumed in the off-gas combustor (i.e., an amount required by the reformer to generate a thermal energy).
Here, of hydrogen generated by the chemical reacting section (the reformer), hydrogen that is not utilized in the power generating operation (an electrochemical reaction) and the hydrogen generating operation (combustion of the off-gas) cannot be discharged to the outside of the system as it is. Therefore, this hydrogen must be burned in, e.g., a residual gas burner to be converted (consumed) into water.
Therefore, the power supply system has a problem that a part of the hydrogen generated from the power generation fuel is wastefully consumed, thereby lowering a power generation efficiency (an energy conversion efficiency). Further, the residual gas burner and its peripheral devices (e.g., a valve or a flowmeter) must be additionally provided, and hence the power supply system also has a problem of an increase in a system scale, complication of control, an increase in a product cost and others.